Exercise-induced changes in expression and activity of proteins involved in insulin signal transduction in skeletal muscle: Differential effects on insulin-receptor substrates 1 and 2

Level of physical activity is linked to improved glucose homeostasis. We de termined whether exercise alters the expression and/or activity of proteins involved in insulin-signal transduction in skeletal muscle. Wistar rats sw am 6 h per day for 1 or 5 days. Epitrochlearis muscles were excised 16 h af ter the last exercise bout, and were incubated with or without insulin (120 nM). Insulin-stimulated glucose transport increased 30% and 50% after 1 an d 5 days of exercise, respectively. Glycogen content increased 2- and 4-fol d after 1 and 5 days of exercise, with no change in glycogen synthase expre ssion. Protein expression of the glucose transporter GLUT4 and the insulin receptor increased 2-fold after 1 day, with no further change after 5 days of exercise. Insulin-stimulated receptor tyrosine phosphorylation increased 2-fold after 5 days of exercise. Insulin-stimulated tyrosine phosphorylati on of insulin-receptor substrate (IRS) 1 and associated phosphatidylinosito l (PI) 3-kinase activity increased 2.5- and 3.5-fold after 1 and 5 days of exercise, despite reduced (50%) IRS-l protein content after 5 days of exerc ise. After 1 day of exercise, IRS-2 protein expression increased 2.6-fold a nd basal and insulin-stimulated IRS-2 associated PI 3-kinase activity incre ased 2.8-fold and 9-fold, respectively. In contrast to IRS-l, IRS-2 express ion and associated PI 3-kinase activity normalized to sedentary levels afte r 5 days of exercise. Insulin-stimulated Akt phosphorylation increased 5-fo ld after 5 days of exercise. In conclusion, increased insulin-stimulated gl ucose transport after exercise is not limited to increased GLUT4 expression . Exercise leads to increased expression and function of several proteins i nvolved in insulin-signal transduction. Furthermore, the differential respo nse of IRS-l and IRS-2 to exercise suggests that these molecules have speci alized, rather than redundant, roles in insulin signaling in skeletal muscl e.